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Autotransformer

 

  • What is preferred for a neutral grounding transformer

      It is up to the user to specify this. Typically, The most common magnetic grounding device is a zig-zag autotransformer. This design offers greater flexibility at a cost and size smaller than a comparable Wye-Delta isolation transformer.

  • What are Motor Starting Autotransformers?

      Motors have a large inrush current upon energization that can stress the electrical system and cause low voltage conditions. Motor Starting Autotransformers (MSAT’s) are used in reduced voltage starters to temporarily reduce the voltage being applied to the motor. This will extend the time it takes the motor to reach full speed and reducing the overall startup current to the motor.

  • What is an Autotransformer?

      It is a transformer that has only one winding per phase, part of which is common to both the primary and secondary circuits.

      Transformers wired in a “Buck-Boost” configuration are autotransfomers. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. An autrotransformers can be used in either a step-up or step-down application unlike isolation transformers. Autotransformers can also be used as part of a reduced voltage starter to reduce motor inrush currents.

  • What is the difference between a buck-boost transformer and an autotransformer?

      A Buck-Boost transformer is typically a small single-phase low voltage lighting transformer that can be wired as an autotransformer to provide small voltage corrections for single and three phase applications. An autotransformer is a transformer with a direct connection between the primary and secondary and does not act as an isolation transformer. Autotransformers can also include wider classes of products including buck-boost, dedicated three coil distribution style units, motor starting autotransformers and solar grid tie transformers.

  • When a Buck-Boost transformer is connected as an autotransformer, what is the procedure for determining the current rating of the over-current protective device, such as the fuse or circuit breaker?

      The NEC Article 450-4 outlines over-current protection for autotransformers. It is reproduced as follows: “NEC 450-4 – Autotransformers 600 Volts, Nominal, or Less

      (a) Over-current Protection. Each autotransformer 600 volts nominal, or less shall be protected by an individual over-current device installed in series with each ungrounded input conductor. Such over-current device shall be rated or set at not more than 125 percent of the rated full load input current of the autotransformer. An over-current device shall not be installed in series with the shunt winding.

      Exception: Where the rated input current of an auto transformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker; the next higher standard rating described in our section shall be permitted. When the rated input current is less than 9 amperes, an over-current device rated or set at not more than 167 percent of the input current shall be permitted.

      (b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as autotransformers shall be identified for use at elevated voltage.”

      Example: A 1kVA transformer, Catalog No. Q1C0ERCB, is rated 120 x 240 to 12 x 24 volts. It is to be connected as an autotransformer to raise 208 to 230 volts single-phase. When connected as an autotransformer in this application, the kVA rating is increased to 9.58 kVA, or 9,580 VA. This is the rating to be used for determining the full load input current and the corresponding size of the over-current protection device, either a fuse or breaker.

      Full load input amps = 9,580 Volt Amps = 46 Amp, 208 Volts.

      When the full load current is greater than 9 amps, the over-current protection device (usually a fuse or nonadjustable breaker).  Current rating can be up to 125 percent of the full load rating of the autotransformer input current.

      Max. current rating of the over-current device = 46 amps x 125% = 57.5 amps.

      The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the over-current device. This is shown in Article 240-6 of the N.E.C.

      Max. size of the fuse or circuit breaker = 60 amps.

  • Why is the isolation transformer kVA rating shown on the nameplate instead of the autotransformer kVA rating?

      Shipped as an isolating transformer, the nameplate is required to show the performance characteristics accordingly. Additionally, as an autotransformer, the eight different combinations of voltages and kava’s would be impractical to list on the nameplate. A connection chart, listing the various connections, is included with each unit.

  • Can the TruWave be used on a 600V system

      Typically the TruWave is rated for 480VAC. However, through the use of a 600V to 480V autotransformer between the TruWave and the load, the TruWave can be used on 600V systems. This same setup can also be used on voltages if needed.

  • What is ANSI NETA ATS-2017
  • As an autotransformer, how can a Buck-Boost transformer supply loads significantly higher than its nameplate rating as a low voltage lighting isolation transformer?

      With an autotransformer, only a portion of the current acts as a load on the transformer.  This portion is roughly proportional to the voltage change.  If you increase the voltage from 100VAC to 120VAC, you are roughly adding 20% (20/100).  As a result, only about 20% of the current acts as a load on the transformer.  This would mean that a 500VA low voltage lighting transformer used in an autotransformer (Buck-Boost) application like this could provide a 2500VA (2.5kVA) load even though the nameplate is only rated for 500VA (.5kVA)

      This is a function of changing the voltage by a small amount. For example, if the transformer is connected in such a way that 22 volts is added to a 208 volt primary, a 230-volt output will result.  Only a portion of the current goes through a buck-boost autotransformer roughly equivalent to the voltage change.  As a result, if a buck boost transformer changes the voltage by 10%, only 10% of the current (kVA) go through the unit.  Therefore a transformer rated for 1 kVA when used as an isolation transformer could handle a 10 kVA load if it adjusted voltage by 10% because only 10% of the total load would go through the unit.

      Using this example, the calculation for autotransformer kVA is as follows:

      KVA = (Output Volts x Secondary Amps)/1000

      KVA = (230V x 41.67 Amps)/1000 = 9.58 KVA

  • What is the base temperature rise of a transformer

      The base temperature rise of a transformer is the maximum temperature rise at the expected full load capacity. Transformers can be built to run cooler than the base temperature rise, these are typically referred to as lower temperature rise transformers which can either operate in higher ambient temperatures or have additional service factor.

      • 105C Insulation System: 55C Base Temperature Rise
      • 150C Insulation System: 80C Base Temperature Rise
      • 180C Insulation System: 115C Base Temperature Rise
      • 220C Insulation System: 150C Base Temperature Rise

      The Hot Spot Allowance is added the expected ambient temperature and full load temperature rise to get the total expected temperature rise of a transformer.

  • Buck-Boost transformers are almost always installed as autotransformers. Does the National Electrical Code (NEC) permit the use of autotransformers?

      Autotransformers are very common and recognized by all the safety and standard authorities.

      You can refer to N.E.C. Article 450-4, “Autotransformers 600 Volts, Nominal, or Less”, as a reference publication. Item (a) details over-current protection for an autotransformer, and Item (b) covers an isolation transformer being field connected as an autotransformer for a Buck-Boost application.

  • Do Buck-Boost transformers present a safety hazard compared to conventional autotransformers?

      Buck-Boost transformers only change voltage by a small amount, such as 208 to 240 volts. This small increase does not represent a safety hazard. Conventional autotransformers, manufactured as single winding transformers, change much higher magnitudes of voltage, e.g. 480 to 240 volts. In a system where the line is grounded, it is possible to have 480 volts to ground when the expectations are that 240 volts is at the output. For this reason, qualified personnel only should maintain conventional autotransformers.

  • Do I need to connect the neutral and ground my HPS three-phase autotransformer?

      If the application needs a neutral (including 3 phase 4 wire systems), the autotransformer must be ordered with the optional neutral terminals (“3L0U” suffix).

      This option will provide the customer with a common (H0/X0) neutral connection point that is connected by the factory to the middle point of the Y winding configuration.

      When selecting this option, both the Line and Load side neutral cables must be connected to the respective neutral terminals in order to ensure the proper operation of the autotransformer.

      HPS does not recommend that the transformer H0/X0 point be grounded locally.

      When an autotransformer without neutral connections is selected, typically the neutral is grounded at the source transformer secondary and is properly referenced throughout the whole installation and carried through to the end load downstream the autotransformer.

      When installing an autotransformer with neutral connections problems can occur when the X0 point of the autotransformer is grounded locally. In such cases a multiple grounding situation may occur which would be against the electrical codes in North America.

      In the above case typically the upstream transformer secondary is grounded at the X0 point of the Y secondary (GND1), in the meantime grounding the X0 point of the autotransformer would create a secondary ground (GND2). Since the two grounds are typically in two different locations, likely far away from each other they will be at different ground potentials.

      This situation can create a number of issues including:

      With the two grounds at different potentials, if the autotransformer center point (X0) is used as a neutral, the line voltages compared to that local neutral would be unbalanced. The extent of the unbalance would depend on the extent of the potential difference between the two grounds (GND1 and GND2). This unbalance could cause issues with the equipment connected to the autotransformer.

      Grounding the X0 of the autotransformer will force the center point of the Y to be always at a certain potential, defined by the local ground. However the voltages of the lines coming into the autotransformer are referenced to the ground point of the upstream transformer. The likely scenario is that the two grounds will be at different potentials which will result in conflicting reference points at the autotransformer. The autotransformer and the electrical system will try to resolve the conflict and equalize the two ground points. The only way that can happen is by having ground current flowing between the two grounds.  Depending on how much of a difference in voltage potential there is between the two grounds and also depending on the ground resistances, there can be a significant current flow through the wye center points.  Adding to this fact that the impedance of an autotransformer is typically low, there could be enough current through the autotransformer to burn out one or more coils of the autotransformer.

      The effects and resulting problems that occur due to improper grounding can be unpredictable and manifest themselves differently in time. Ground potentials can greatly vary depending on environmental conditions.  After installing an autotransformer and grounding the center point of the wye (X0) problems may not surface initially.  However, there is a chance that after a rainstorm or some other event, all of a sudden the user experiences high ground currents just because the grounding conditions have changed.  These problems could be very intermittent in nature and hard to diagnose.

      When an autotransformer with neutral connections is requested, we do not recommend the grounding of the X0 point and recommend that the customer and installing contractor should refer to the local electrical code requirements for grounding and the short circuit protection of a three phase autotransformer.

  • How do I use a Grounding Transformer

      Three-phase grounding transformers provide an artificial neutral for grounding. The main requirement is a specific zero-sequence impedance from the Zig-Zag or the Wye/Delta transformer in addition to the fault current withstand rating. For grounding purpose, only the Zig-Zag or a Wye/Delta connected transformer can be used. Autotransformers will have a high zero-sequence impedance and hence, cannot be used for grounding. Air-core reactors can be normally connected between the artificial neutral and ground to provide some additional current-limiting impedance.

      Grounding transformers are often required by the utility to attach a load generator such as solar, wind or a generator to the power grid.